Fingerprint has almost become a pronoun for biometric features identification due to the lifelong invariance, uniqueness and accessibility. While a fingerprint identification technology has also been widely applied to various terminal devices, such as a mobile terminal, a banking system, an attendance system, or the like, and is frequently used for providing security access to sensitive electronic devices and/or data.
A capacitive fingerprint detection circuit is a front analog circuit that is operable to read fingerprint information. FIG. 1 shows a circuit structure of a known design, and its principle is to conduct a driving signal (for example, a driving signal with a frequency of 400 KHz) onto a touch plane; when a finger touches the plane, a touch operation can transmit a fingerprint sensing signal into a system through a capacitor on the plane.
Chinese invention patent application CN103376970 A discloses another capacitive fingerprint sensor, where the capacitive fingerprint sensor can be formed by an array of sensing elements. Each capacitive sensing element of the array records a voltage that changes with the capacity coupling capacitance, and the capacitive sensing element is located between the finger and a sensing chip, and the finger can be capacitively coupled to each capacitive sensing element of the sensor, so that the sensor can sense the capacitance between each capacitive sensing element and a fingerprint muscle. The sensing chip detects a capacitance signal through sensing the voltage changes on the capacitive sensing element. Also, the sensing chip can detect a capacitance signal by sensing the change of charges that are received by the capacitive sensing element.
There is a problem in the two fingerprint identification technologies described above and in other existing designs that because the capacitance C in the feedback loop of an amplifier is very small, it is very difficult to amply the fingerprint sensing signal and the SNR (signal-to-noise ratio) is low.